Non-Linux FOSS: Open-Source Windows?
I have mixed emotions about ReactOS. It's open source. It's freely available. But, its goal is to be binary-compatible with Windows! ReactOS is not a Linux operating system. In fact, it doesn't share the UNIX architecture at all. It looks like Windows NT, and it behaves much like Windows NT.
It's just odd!
The best way I can think to describe it is to imagine if Wine evolved into an entire operating system that booted on hardware instead of running inside Linux. That's basically what ReactOS feels like. It's not ready for prime time (and the developers make that very clear—it's alpha software), but it's worth checking out. Since it's early in the development process, if you get involved now, you can have a say in what compatibilities get priority.
ReactOS is the perfect solution for folks who need to run Windows apps, but absolutely refuse to run Microsoft code. I'm personally not convinced that ReactOS is a better idea than Wine running inside Linux, but I'm sure running it as its own operating system will provide possibilities that just can't happen in a Wine environment. The folks at ReactOS provide installers and prebuilt VM instances that can be launched in order to try it out on your existing system. Whether you are just morbidly curious about a non-Windows Windows or are interested in getting involved in the development, go to http://reactos.org for more details.